SMA connector on a microstrip Line

Hi,

Please would you experts help check the attached for a simple 50Ω microstrip line fed by a SMA connector simulated in HFSS. Both the microstrip line and SMA coaxial dimensions were separately simulated and matched, but rubbished when connected together. The entire structure behaves like a resonator rather than a simple matched line through the frequencies. Did I forget to set up anything? Thanks very much.

For me the S21 = -70dB at 2GHz looks like the central pins of the SMA and the microstrip transmission line are not united, and there is just a small capacitive coupling. Or there is no ground connection between SMAs.

The resonances at different microwave frequencies make sense for a relative long 50 ohms microstrip line, but these resonances should appear on S11, and not on S21.

Hi vfone,

Thanks for your reply. The central pins and probes of the connectors and the microstrip lines were united as a whole, and PEC boundary was assigned. The probes are intercepted into the microstrip line and united. Agreed what you commented that there may be capacitive gaps existed that may produce a response like that, but there were not any gaps in the layout. Is it something about the port setting, or the way we short the SMA ground leads onto the ground plane of the board, or any other drawing mistakes?

If you short to the ground the input of the SMA, there will be even worst results.

What abut ground connection between connectors?

Try to remove the SMA connectors, place the ports at the edges of the microstrip line, and check the results.

Hi vfone,

Thanks very much. It works now as I realised we need to set up an air box with radiation boundary enclosing the entire structure while others remain unchanged. Please refer to the attached for the results.

Now is fine. As I mentioned before, the resonances you can see only on S11, which is normal.

Hi chunweimin. I am a beginner in HFSS use and I would like to simulate a microstrip line fed by a SMA connector in HFSS. Please can you tell me what steps you had followed for your simulation.

Thanks very much